Medical devices that infuse liquids into patients are called infusers. One type of infuser uses an elastomeric bladder as its power source. Such infusers and bladders are described in U.S. Pat. Nos. 3,993,069 and 4,201,207. These infusers consist of a housing, an elastomeric bladder contained within the housing that is inflated with the liquid to be infused, and a conduit that leads from the bladder to the infusion site. The rate at which the liquid is infused from the infuser depends upon the pressure exerted on the liquid by the bladder, the viscosity of the liquid, and the flow restriction characteristics of the conduit. The above cited patents describe bladders that are capable of maintaining the pressure on the liquid substantially constant over discharge of a large proportion of the liquid. The bladders described in these patents are made from vulcanized synthetic polyisoprene that has a low frequency hysteresis less than about 10% and a stress relaxation less than about 10%. Such hysteresis and stress relaxation characteristics were considered as key factors in realizing substantially constant pressure performance.
In making large numbers of such bladders from synthetic polyisoprene it was found that a small but significant number of them ruptured when inflated--particularly after prolonged storage in an inflated state. Even though only a small portion of the bladders so ruptured it was desirable to decrease the incidence of rupture in order to provide a greater margin of safety against rupture in the marketplace. The above patents say nothing about reducing the incidence of bladder rupture.
A principal object of the present invention is to provide synthetic polyisoprene bladders that have a reduced incidence of rupture and acceptable pressure constancy performance.